Scientific Application
Packers
A “packer” is an inflatable rubber element that inflates to seal
the annular space between the drill string and the borehole wall. While there
are different types of packers, ODP runs one, or two in tandem assemblies, on
drill string into a pre-existing ~9
in. hole (cored, drilled, or cased) and
inflates it with seawater to hydraulically isolate a borehole section from the
annulus and tidal effects of the open ocean. Isolating the borehole from the
open ocean allows measurement of pore pressure and in situ hydrological
properties of the formation. Formation fluids may be sampled if the formation is
permeable. It is possible to derive formation permeability and porosity in an
isolated open-hole section using a straddle packer (i.e., two packers) by
applying pump pressure with pulse, slug, or constant-rate injection tests.
The 8¼ to 8½ in. outer diameter (OD) packer element can be
expanded to as much as twice the uninflated diameter (~16 in.); however, full
inflation produces a weaker hydraulic seal that withstands less differential
test pressure. Effective formation testing in an open hole requires that all
packers be positioned in low-permeability borehole sections that are relatively
in-gage and have smooth bores. Wireline logs such as caliper, borehole
televiewer, resistivity, density, and sonic logs are typically run before packer
tests to identify permeable test intervals and good seal sections. The logging
information allows the bottom-hole assembly (BHA) and packer(s) to be spaced out
accordingly. Logs and experiments can be run through an uninflated packer;
however, wireline operations through an inflated packer require special safety
measures because the packer grips the borehole wall. The packer immobilizes the
BHA, but the top of the pipe at the rig floor can still move up and down,
requiring drill string heave compensation to avoid tearing the packer loose.
The drill string packer can be run in several combinations:
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A single-element packer is used to isolate the interval
below the packer to the bottom of the hole.
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Two packers can be run together, acting as a single packer
to increase the seal length. Pressure gages can be run to confirm the seal.
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A “straddle packer” incorporates two separate packers to
span the test interval so it can be sampled or exposed to injection tests.
The packers are separated by 1 to 9 m long drill pipe pup joint(s). The
interval between the straddle packers and the interval from the bottom
packer to the bottom of the hole can be tested separately. The straddle
packer elements are connected by
in. stainless steel tubing (strapped on
the outside the drill pipe spacer) to ensure that neither element can be
inflated if there is a hydraulic failure. A pressure recorder can be hung
below the lower Go-Devil to test for leakage from the straddled interval.
Flowmeters
A flowmeter is run on the wireline to measure the flow rate of
the fluid pumped into the hole. The flow rate can be measured throughout the
open hole. Within cased hole, the flow rate is 100%. Below the casing, flow
rates decrease as a portion of the fluid flows into the formation, providing
information about permeability in the borehole (i.e., flow rate decreases below
high permeability sections).
Tool Operation
Packer Operation
The packers typically used by ODP are made by TAM International,
Inc., of Houston, Texas. The 8¼ in. TAM packer assembly consists of an internal
steel strength member, an expandable internal rubber bladder, and an 8¼ to 8½
in. OD outer rubber cover. A Go-Devil plug is dropped down the drill pipe and
lands in a seating nipple in the pipe (see Packer Control Sub figure). The
Go-Devil seals the drill pipe so pump pressure can inflate the packer bladder
with seawater pumped down the drill string. The seawater presses the rubber
bladder outward to grip and seal against the borehole wall. The bladder cover
has an embedded expandable (woven stainless steel) strength member.
Drill string packer(s) are run on the drill string as part of
the BHA and are positioned over the desired interval based on logging depths.
When the drill string is freely suspended without heave compensation (running in
and out of the hole), the packer element(s) remain uninflated. When the packer
is in position, a wireline retrievable Go-Devil with two GRC (vendor) pressure
recorders (covered with a special core barrel and retaining catcher) is
free-fall deployed and pumped down the drill string at ~200-250 gpm to increase
the speed. The Go-Devil lands in a “seating nipple” at the packer, and a
hydrostatic pressure baseline is recorded for 10-15 min.
The rig pumps are typically used to inflate the packer element(s)
with seawater to ~1200 to 1500 psi maximum. When the packer elements inflate and
start to grip the borehole wall, air is bled off the heave compensator, which
lowers the drill string to set down ~15,000 lb weight on the packer (~12,000 lb
of weight/1000 psi of test pressure) to prevent packer movement from the piston
effect of test pressures.
To inflate and deflate a packer, a Packer Control Sub is used.
It also provides a way to run a pressure instrument in the hole. The Packer
Control Sub has an internal “control tube” that connects two or more packers
hydraulically. Lowering the drill string ~20 cm (8 in.) shifts the packer
control sleeve down, isolates the inflated packer element, and opens the bore to
the interval below the upper packer. The heave compensator is positioned at
mid-stroke, and ~15-20 min is required for the pressure pulse to decay. A
typical set-down weight of 15,000 lb requires a BHA of approximately six 8¼ in.
drill collars above the packer. ODP recommends running several drill collars
below the packer to help set it. The packer elements are not designed to support
much more than ~15,000 lb weight.
Formation fluids may be sampled before pump tests if the
formation is permeable enough. Go-Devils can be used to activate multiple
packers and open or close valves for formation testing. The Go-Devil and
pressure recorders are retrieved with the coring wireline.
After formation testing is completed, the packer(s) are deflated
by using the heave compensator to pull the drill string upward and open the
deflation ports. The straddle packer element deflates passively in ~30 min, and
it can be moved and reset (~2-3 times) if the element is not damaged.
When straddle packers are used, pulse, slug, or constant-rate
injection tests may be run to determine the formation transmissivity (from which
permeability can be derived) and (less accurately) storage coefficient. Storage
coefficient is directly related to formation porosity. The cementing-unit pump
is typically used to apply pressure (~500 to 1500 psi) to the isolated open-hole
section. Test pressures typically do not exceed 1500 psi (unless formation
hydro-fracture is attempted), and pump rates typically do not exceed 3-5
bbl/min. All pumped (injection) volumes should be measured using a standpipe
meter or the volume tanks on the cementing unit. Pressure pulses may require up
to 30 min to decay, and multiple tests (2 to 4) may be run.
Flowmeter Operation
A flowmeter/sinker bar/Go-Devil assembly can be run on the
logging wireline. After inflating the packer, the logging line is sheared off
the Go-Devil by pressuring up to 2000 psi. The flowmeter is lowered below the
packer (~50 m) and calibrated by pumping at several rates. Flowmeter readings
typically are taken at 10 m intervals (to total depth [TD]) for ~10-20 min at
each interval. The flowmeter/Go-Devil assembly is retrieved by wireline.
Design Features
1) Compatibility
The drill string packer BHA is compatible with wireline logging
tools.
Benefit: It may be possible to accomplish both
logging and packer measurements during a single pipe trip.
2) Multiple Tests
A packer can be set, moved, and reset (up to three times) if the
element is not damaged.
Benefit: Multiple tests can be attempted during one
trip in the hole.
Specifications
Packer
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TAM International, Inc., model TAMJAY Packer, 8¼ in. OD
element, stroked out length 3.12 m (10.2 ft), 3.97 in. internal diameter
(ID), connection 5½ in. IF box up and pin down
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Minimum ID: Single packer 3.94 in., Straddle nonrotating
packer 3.84 in., and rotating packer 3.84 in.
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Packer Inflation Pressure: typically ~500 to 1000 psi (~half
of planned test pressure), 1500 psi maximum
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Set down weight on packer: ~12,000 lb weight/1000 psi test
pressure, 15,000 lb maximum
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Drill string movement to shift control tube: ~20 cm (8 in.)
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Typical Packer BHA: 9 to 9B\, in. bit, crossover, 1 joint 5½
in. drill pipe, 8¼ in. packer, crossover, ~5 each 8¼ in. OD drill collars,
tapered drill collar, crossover to 5 in. drill pipe. BHA length ~70 m,
weight ~18,000 lb
Pressure Recorders
The GRC pressure recorder is typically set for 10,000
measurements per day. It is run below the Go-Devil in a special core barrel
tube with a retaining catcher.
Kuster model K-3 mechanical self-contained pressure
recorders are available in three pressure ranges (0-9925, 0-11900, and
0-15275 psi) and can record for 9, 18, or 36 hours. For a single packer, two
K-3 recorders are free-fall deployed inside a 12 ft long inner core barrel
tube attached to the Go-Devil. For straddle packers, one K-3 recorder is
attached to the lower Go-Devil and one or two to the upper Go-Devil.
A Gould PG3000 pressure transmitter (0-5000 psi) is
installed at the standpipe to provide a real-time pressure record.
Typical Operating Range
Packer elements and seals are rated to ~100°C. Special
high-temperature elastomers (i.e., packer elements and inner seals) rated to
120°C were used (Leg 111 – Hole 504B) at temperatures of 120°-145°C, but
lasted for only one setting.
Limitations
Packer Damage
The rubber elements on packers are made of elastomers that can
be damaged by contact with jagged formations. Moving a set packer before it has
relaxed back to normal size may damage the element.
Hole Damage from Packers
The packer expands with hydraulic force to firmly contact the
formation; however, over-inflation can fracture the formation or burst the
packer element. Packers can also reduce the hydrostatic pressure, causing a
formation to flow or cave-in, resulting in stuck pipe.
Temperature
The packer elements are made of an elastomer that has an
operating temperature range of 100°-120°C.
TAM Nonrotating Packer
The TAM drill string straddle packers are a nonrotating design
that is deployed with the drill string. They should be kept in tension and are
not compatible with a coring and rotating BHA; therefore, a separate trip is
required to run the packer.
TAM Rotatable Packer
The TAM rotatable packer was designed for use as a single packer
for testing and drilling ahead; however, tests (Legs 110, 123, and 130) indicate
that it may be more suitable for tests in unstable pre-drilled reentry holes
that may require some reaming or cleanout.
(April
2005)